1. Field of the Invention
The present invention relates to an electric rotating machine having a stator in which a plurality of magnetic poles comprising permanent magnets are arranged on the inner circumferential surface of a yoke. The present invention may be used for a starter for an engine.
2. Description of Related Art
Conventionally, in a stator using permanent magnets as magnetic poles, the thickness of a yoke made of a steel plate is made thinner than that of a yoke of a stator of coil winding type. Thus, this stator using permanent magnets is preferable for manufacturing an electric rotating machine in compact size and in light weight. However, when a thin yoke is used for a starter which is used under high vibration conditions, the yoke may resonate with vibrations transmitted from external parts such as an engine because the rigidity of the yoke is low. In this case, through-bolts which are tightening members loosen. As a result, abnormal wear or the like may be caused in component parts.
In U.S. Pat. No. 5,475,276 which corresponds to Japanese Laid-Open Patent Publication No. 8-163798, there is proposed an electric rotating machine in which, as shown in FIG. 4, between magnetic poles (permanent magnets) arranged circumferentially adjacent to each other, radially inwardly extending concaves 6 are formed on the cylindrical wall portion of a yoke 2 from the outer circumferential side thereof to the inner circumferential side thereof, and through-bolts 4 are installed in the concaves 6. In this case, because the concaves 6 perform the action of a rib, the tightening force of the through-bolts 4 can be received by the end surfaces of the concaves 6. Thus, a sufficient rigidity can be securely provided even though the thickness of the yoke 2 is reduced.
However, when the concaves 6 are formed on the cylindrical wall portion of the yoke 2, each of the concaves 6 is interposed between the magnetic poles which are adjacent to each other circumferentially. Thus, the distance between the side surface of each of the magnetic poles and the yoke 2 (concave 6) is shorter than the distance between the side surface of each of the magnetic poles and the yoke 2 having no concaves formed on the cylindrical wall portion thereof. As a result, the magnetic leakage between the side surface of each of the magnetic poles and the yoke 2 (concave 6) increases and thus the performance of the electric rotating machine is lessened.